Recently, short and thin cantilevers have been used for atomic force microscope (AFM) imaging and force measurement [D. A. Walters, J. P. Cleveland, N. H. Thomson, P. K. Hansma, M. A. Wendman, G. Gurley, V. Elings, Short cantilevers for atomic force microscopy, Rev. Sci. Instrum. 67, 3583 (1996); M. B. Viani, T. E. Schaffer, G. T. Paloczi, L. I. Pietrasanta, B. L. Smith, J. B. Thompson, M. Richter, M. Rief and H. E. Gaub, K. W. Plaxco, A. N. Cleland, H. G. Hansma, P. K. Hansma, Fast imaging and fast force spectroscopy of single biopolymers with a new atomic force microscope designed for small cantilevers, Rev. Sci. Instrum. 70, 4300 (1999)]. Such cantilevers have a high resonance frequency and a low force constant. The high resonance frequency allows for fast imaging [M. B. Viani, T. E. Schaffer, G. T. Paloczi, L. I. Pietrasanta, B. L. Smith, J. B. Thompson, M. Richter, M. Rief and H. E. Gaub, K. W. Plaxco, A. N. Cleland, H. G. Hansma, P. K. Hansma, Fast imaging and fast force spectroscopy of single biopolymers with a new atomic force microscope designed for small cantilevers, Rev. Sci. Instrum. 70, 4300 (1999); M. B. Viani, L. I. Pietrasanta, J. B. Thompson, A. Chand, I. C. Bebeshuber, J. H. Kindt, M. Richter, H. G. Hansma, P. K. Hansma, Probing protein-protein interactions in real time, Nat. Struct. Biol. 7, 644 (2000)], which is highly desirable for monitoring the reactions, interactions, and conformational changes of biomolecules. The low force constant of the cantilever greatly minimizes the deformation of soft samples such as biomolecules. In addition, it significantly increases the sensitivity of force measurement which has been widely used for studying the inter- and intra-molecular interactions between macromolecules [M. B. Viani, L. I. Pietrasanta, J. B. Thompson, A. Chand, I. C. Bebeshuber, J. H. Kindt, M. Richter, H. G. Hansma, P. K. Hansma, Probing protein-protein interactions in real time, Nat. Struct. Biol. 7, 644 (2000)]. Furthermore, it has been demonstrated that small cantilevers also reduce thermal noise [D. A. Walters, J. P. Cleveland, N. H. Thomson, P. K. Hansma, M. A. Wendman, G. Gurley, V. Elings, Short cantilevers for atomic force microscopy, Rev. Sci. Instrum. 67, 3583 (1996)]. All these unique advantages associated with small cantilevers have motivated the development of processes for the fabrication of such cantilevers and the instrumentation for using them.
Currently, AFM cantilevers are predominately fabricated from silicon and silicon nitride (SiN). Several reports for the fabrication of small cantilevers without tips using both materials appeared [T. D. Stowe, K. Yasumura, T. W. Kenny, D. Botkin, K. Wago, D. Rugar, Attonewton force detection using ultrathin silicon cantilever, Appl. Phys. Lett. 71, 288 (1997); J. Yang, T. Ono, M. Esashi, Mechanical behavior of ultrathin microcantilever, Sens. Actuator, A. 82, 102 (2000); A. Gupta, J. P. Denton, Helen McNally, R. Bashir, Novel fabrication method for surface micromachined thin single-crystal silicon cantilever beams, J Microelectromech. Syst. 12, 185 (2003)]. Among the few reports for the fabrication of small cantilevers with tips, most of them used low stress SiN as the material for the cantilevers because the thickness of SiN cantilevers can be controlled by chemical vapor deposition (CVD) and SiN is not etched in the etchants for silicon or silicon oxide [M. B. Viani, T. E. Schaffer, A. Chand, M. Rief, H. E. Gaub, P. K. Hansma, Small cantilevers for force spectroscopy of single molecules, J Appl. Phys. 86 2258 (1999)]. The SiN tips can be fabricated with SiN cantilevers together or carbon tips can be deposited on the SiN cantilevers by electron beam deposition [K. I. Schiffmann, Nanotechnology 4, 163 (1993)]. Although silicon cantilever tips are more difficult to fabricate compared to SiN cantilever tips, there are several advantages for the use of silicon tips and cantilevers: 1) Si tips can be easily sharpened through thermal oxidation; 2) Comparing with carbon tips deposited by electron beam deposition, the Si tips that are integrated with Si cantilevers are easier to be fabricated in large scale; 3) Si tips can be modified with protein-resistant monolayers for application of such tips for imaging biological samples [Yam, C. M., Lopez-Romero, J. M.; Gu, J.; Cai, C. “Protein-resistant monolayers prepared by hydrosilylation of □-oligo(ethylene glycol)-□-alkenes on hydrogen-terminated silicon (111) surfaces.” Chem. Commun. 2510-2511 (2004); Yam, C. M.; Xiao, Z.; Gu, J.; Boutet, S.; Cai, C. “Modification of silicon AFM cantilever tips with an oligo(ethylene glycol) derivative for resisting proteins and maintaining a small tip size for high-resolution imaging.” J. Am. Chem. Soc. 125, 7498-7499 (2003); Gu, J.; Yam, C. M.; Li, S.; Cai, C. “Nanometric protein arrays on protein-resistant monolayers on silicon surfaces.” J. Am. Chem. Soc. 126, 8098-8099 (2004)]; 4) The single crystal Si cantilevers have a higher Q factor than the amorphous SiN cantilevers grown by CVD [Yam, C. M.; Xiao, Z.; Gu, J.; Boutet, S.; Cai, C. “Modification of silicon AFM cantilever tips with an oligo(ethylene glycol) derivative for resisting proteins and maintaining a small tip size for high-resolution imaging.” J. Am. Chem. Soc. 125, 7498-7499 (2003)]; 5) At an appropriate thickness, the Si cantilevers thinner than 500 nm have as high as 70% reflectivity to red laser, which is a popular light source in commercial SPM heads. However, the corresponding SiN cantilevers only have up to 40% reflectivity to red laser. Moreover, if blue-violet laser is used in SPM heads, the reflectivity in thin Si cantilevers has almost no variation with thickness because the strong absorption of blue light nearly completely eliminates the thin film interference for Si. In contrast, SiN almost has no absorption to visible light.
Despite the advantages of using Si tips and cantilevers, few reports on the fabrication of thin Si cantilevers with Si tips and only one report on the fabrication of Si tips on SiN cantilevers appeared [R. J. Grow, S. C. Minne, S. R. Manalis, C. F. Quate, Silicon nitride cantilevers with oxidation-sharpened silicon tips for atomic force microscopy, J Microelectromech. Syst. 11, 317 (2002)]. This situation reflects the difficulties of fabricating thin Si cantilever tips. The commercial Si cantilever tips are mostly prepared by backside etching of 300-500 μm thick Si wafers. Due to the deviation of thickness over the whole wafer, and the difficulties in controlling the etching process, it is extremely difficult to prepare cantilevers thinner than 1 μm in a wafer scale by this process. The excellent performance of cantilever tips made of SiN with sizes similar to a few microns width, down to 100 nm in thickness, and around 10 μm in length has been demonstrated [M. B. Viani, T. E. Schaffer, G. T. Paloczi, L. I. Pietrasanta, B. L. Smith, J. B. Thompson, M. Richter, M. Rief and H. E. Gaub, K. W. Plaxco, A. N. Cleland, H. G. Hansma, P. K. Hansma, Fast imaging and fast force spectroscopy of single biopolymers with a new atomic force microscope designed for small cantilevers, Rev. Sci. Instrum. 70, 4300 (1999)]. However, to take advantage of such micron-sized cantilevers requires specially designed AFM heads that remain at the prototyping stage and are available in only a few research groups.